ANDRZEJ PEKALSKIDelft University of Technology

Julianalaan 1362628 BL Delft

The NetherlandsE-mail: Pekalski@tnw.tudelft.nl

http://www.dct.tudelft.nl/part/explosion/

Proper determination of the upper flammability limit at

elevated conditions (high temperature andhigh pressure)

Dr NICO DAMUniversity of Nijmegen

Toernooiveld 16525 ED NijmegenThe Netherlands

SOCIETAL NEED – industrial processesPartial oxidation processes and their conditions in chemical industry

Final productAnnual worldproduction

(106 tonnes/year)Temp.

(°C)Pressure

(bars)Acetic acidFrom: acetaldehyde, alkanes,alkenes, light gasoline, methanol

6.0 (1994) 50 ÷ 200 15 ÷ 80

AcetaldehydeFrom: ethylene, ethanol

2.4 (1993) 100 ÷ 460 3 ÷ 20

Ethylene oxideFrom ethylene

11.2 (1995) 200 ÷ 300 10 ÷ 30

Propylene oxideFrom propylene

4.0 (1993) 90 ÷ 140 15 ÷ 65

Maleic anhydrideFrom: benzene, butene, butane

0.87 (1995) 350 ÷ 500 2 ÷ 5

Phtalic anhydrideFrom naphtalene, o÷xylene,butane

2.9 (1995) 150 ÷ 550 1 ÷ 3

Precise determination of the upper explosion limits enables:

• Safe operation (most desired outside the flammable range)

• Process optimisation (reduction of un-necessary error margin)

• Productivity increase

SOCIETAL NEED – safe and efficient operation

International standards on flammability limits

At elevated temperature and elevated pressure• ASTM E918-83 (1999); up to 200 oC and 138 bara

ignition criterion (B): Pexp/Pinit > 7%

At elevated temperature and atmospheric pressure• ASTM E 681-01; up to 150 oC

ignition criterion: flame detachment• DIN 51 649, part 1; up to 200 oC

ignition criterion: flame detachment• prEN 1839 (T-tube) and (B-bomb); up to 200 oC

ignition criterion (T): flame detachmentignition criterion (B): Pexp/Pinit > 5%

Cool flames are observed for most hydrocarbonsCool flames

• Present in fuel rich mixtures, at elevated conditions

•Higher pressure enhances its occurrence

Comparison between cool flames and hot flames

Cool Flames Normal flames Flammability range Wide Narrow Heat liberation Low High ∆T [deg] (in air) 10-150 (400) 1600-2800 Temperature coefficient Negative Positive Pf/P0 (in confined spaces) Below 2 (low) 6-10 (high) Flame velocity [cm/s] 3-5 30-325 Degree of conversion Low Completely Products HCHO, CO H2O, CO2

Problem Formulation1. How to distinguish between the UEL and the LCFL

at elevated conditions in a closed volume?

2. What kind of flame is propagating at very fuel rich compositions?

Research equipment

Tmax=300oC, Pmax= 275 barFlame emission spectroscopy was applied

Experimental programme

n-butane/oxygen mixtureTinit = 225 oC, Pinit = 2 bara, varying O2 concentrationTinit = 225 oC, Pinit = 4 bara, varying O2 concentration

CH4/C2H4/O2 mixtureTinit = 225 oC, Pinit = 16 bara

A typical experimental runO2 =27.06 %, C4H10=rest, Pinit= 4 bara, Tini =228 C

2.5

3.5

4.5

5.5

6.5

7.5

8.5

0 20 40 60 80 100 120 140time [s]

Pre

ssur

e [b

ar a

bs]

200

250

300

350

400

450

500

550

Tem

pera

ture

[C]

P2 ign T1 T2 T3 T4 T5 T6

CCD camera image recordExposure time = 5 seconds, max gain, slot 1 mm

N-Butane/oxygen flame (XO2 = 27.06%)

3800

4000

4200

4400

4600

4800

5000

5200

5400

5600

380 385 390 395 400 405 410Wavelength [nm]

Inte

nsity

Flame emissionN-butane/O2 mixture at 4 bara

Intensity coming from chemically excited HCHO (highest 10) : 385.6 (9) 396.0 (10), 405.3 (5) and 412.9 (8)

Returned flame (down going) ONLY

Subtraction of up-going flame from up and down spectra

Up going flame

CH4/C2H4/O2 mixture at 16 bara

Spectrum of the returning flame (experiment 21-20)

2500

2600

2700

2800

2900

3000

3100

3200

3300

3400

380 385 390 395 400 405 410

Inte

nsity

Methane/ethylene flame, experiment 20 (T16-17)

450

500

550

600

650

700

750

800

380 385 390 395 400 405 410Wavelength [nm]

Inte

nsity

• Propagating flame in the sphere, initiated by a fused wire, forms excited formaldehyde

• At fuel rich concentrations at elevated conditions artificial ignition source (fused wire) initiates propagation of cool flame

• Based on the current international standards one may determine upper cool flame limit instead of the upper flammability limit

Conclusions

Any questions are welcome

Pressure records in a closed volume:

a – one cool flame,b – two cool flames,c – two-stage ignition with

intermediatetemperature decrease,

d – two-stage ignition without intermediate temperature decrease,

e – one-stage ignition (hot ignition)

Cool flames appearance